1. Field of the Invention
The present invention is related generally to a method of performing handoffs in a wireless communication system, and more particularly to a method of performing handoffs in wireless local area networks (WLANs).
2. Description of the Related Art
In WLANs, mobile devices use access points (APs) (e.g., a wireless router) to connect to wired networks and communicate with other hosts. An AP is typically equipped with an interface that connects to a wired network (e.g., via an Ethernet connection) and a wireless interface (e.g., IEEE 802.11b, etc.) that communicates with mobile devices.
A coverage area of an AP operating indoors in accordance with 802.11 is limited to approximately 200 to 300 feet. Additional APs may be used to expand the coverage area. When a mobile device moves from the coverage area of a first AP to a second AP, the handling of the mobile device's communications has to be “handed off” from the first AP to the second AP. In 802.11b, handoffs occur at either layer-2 or layer-3 of a protocol stack.
For example, if the first and second APs function as MAC layer (layer-2) bridges, the handoff is performed only at layer-2 because both the first and second APs belong to the same IP subnet. In an alternative example, if the first and second APs function as IP (layer-3) routers, the first and second APs belong to different IP subnets. Thus, in addition to a layer-2 handoff, a layer-3 handoff is also necessary.
In WLANs intended for large coverage areas, seamless roaming support for mobile devices within the network is a desirable design criteria. In other words, ensuring mobility of mobile devices throughout the WLAN coverage area without any service disruptions is desired. Thus, transport and application level sessions should not be disrupted during handoffs between APs. Real-time applications (e.g., VOIP, streaming audio and/or video, etc.) require that handoffs (e.g., layer-2 and/or layer-3 handoffs) be performed fast enough to avoid service disruption. For example, disruptions in a VOIP call would be noticed if the “jitter” is above a time threshold (e.g., 50 milliseconds (ms)). Thus, to support VOIP in 802.11 WLANs and avoid service disruption (e.g., jitter), handoffs must be performed in less time than the jitter time threshold.
Conventional 802.11 WLAN networks perform layer-2 handoffs in accordance with a break-before-make approach, alternatively referred to as “hard handoff”. In hard handoff, a radio card (e.g., a PCMCIA wireless 802.11b card, etc.) on the mobile device begins probing for available neighboring APs with acceptable signal strengths if a signal strength of the connection with the serving AP drops below a signal strength threshold. In an example, 802.11b WLANs include 11 channels which may be probed for available APs, where a probing of all 11 channels may take up to a second to complete. Once an acceptable AP is discovered through the probing step, the mobile device authenticates with the new AP and then associates with the new AP by performing a layer-2 association. While still significant, delays associated with the associating and authenticating steps (e.g., 10 ms) are typically less than delays associated with the probing step (e.g., up to 1 second).
The above-described probing, authenticating and associating steps may take a substantial amount of time (e.g., hundreds of milliseconds) which may vary based in part on the type of radio card being used. Further, if required, layer-3 handoffs add additional handoff latencies (e.g., on the order of hundreds of milliseconds). The delays associated with layer-2 and layer-3 handoffs are often large enough to cause service disruption in real-time applications in 802.11 WLANs.
A conventional method of reducing the above-described probing delays includes reporting the presence of neighboring APs to each mobile device in the coverage area of a 802.11 WLAN. Thus, since APs typically remain on the same channel, the probing step may be limited to channels associated with the APs reported to the mobile device. However, the above-described conventional method requires maintenance and dissemination of information to the mobile devices in the coverage area, and further requires changes to 802.11 protocols and an active management of the AP reports.
Another conventional method of reducing the probing delay is referred to as SyncScan where all APs in a 802.11 WLAN are synchronized and configured to output beacons (e.g., similar to pilot signals in cellular systems) on predetermined channels at predetermined intervals. For example, a first AP on channel 1 broadcasts a first beacon at time t, a second AP on channel 2 broadcasts a second beacon at time t+d, and so on. Mobile devices configured for operation with SyncScan switch to the predetermined channels at the predetermined times to attain AP information and then switch back to the serving AP to continue previous communications.
In 802.11 systems operating in accordance with SyncScan, the clocks at each participating AP must be synchronized with each other, which can be very difficult to achieve. Further, clock drift needs to be bounded so that the mobile device tunes to the correct channel at the correct time. Further, it is difficult to transmit beacons precisely at the predetermined times, which may require that the mobile devices operating in accordance with SyncScan tune to the channel longer than expected, thereby decreasing network efficiency. Also, while the mobile device tunes to a channel in accordance with SyncScan, data packets received from a current serving AP are lost and need to be retransmitted when the mobile device switches back to the current serving AP.